This invention relates to a device for supervising impedance-earthed, i.e. non-directly-earthed, power systems. By use of this device, hereinafter referred to as an NX-analyser, a continuous calculation of the attenuation (d), dissymmetry (k) and mismatch (v) parameters of the system from the circle diagram uEN=f(v) is provided.
To the extent that the neutral point voltage criterion is used today for monitoring or control purposes, it is purely the value of amplitude that is measured. Amplitude measurement is used, inter alia, for such purposes as indication of abnormal conditions in the power system, such as, for example single-phase earth faults. Use is also made of the resonance curve, or of the relationship between the amplitude of the neutral point voltage and the degree of mistuning, in devices for automatic tuning of Petersen coils.
On the other hand, the fact that a certain neutral point voltage of varying magnitude can occur even in the faultfree condition does limit the applicability of the amplitude criterion method for general monitoring purposes. Here the NX-analyser effects an improvement. The continuous calculation and processing of the values of the above-mentioned system parameters creates a whole set of new criteria for various monitoring and control purposes in the above systems. The following, for example, can be considered:
1. Degree of mistuning (v): Monitoring of the self-extinguishing condition in accordance with DIN 57288 and compliance with the current limiting requirement in accordance with various legal requirements are important means of monitoring non-directly-earthed high-voltage systems. Existing known means for providing such monitoring are connected for automatic tuning of Petersen coils. The majority of devices utilize as control criterion the resonance curve .vertline.U.sub.EN .vertline.=f(v) and the fact that U.sub.EN reaches its maximum value at the exact point of tuning (v=0).
Another procedure makes use of the relationship .phi..sub.UEN =f(d) where, as known, the angular change falls off with decreasing degree of mistuning (Swiss Pat. No. 532854). However, the common feature of all known tuning procedures is that they require a stepless adjustable neutral point reactor. Monitoring/control is also limited to the control range of the reactor. Such use of a quantitative criterion for the degree of mistuning, independently of the particular coil setting, here provides several benefits. Firstly, no controllable neutral point reactor is required, and secondly, monitoring of the degree of mistuning can continue to operate outside the setting range of the reactor. Use of the NX-analyser also facilitates separation of the monitoring and control functions. In many systems monitoring alone is quite sufficient, and any necessary adjustment of the reactor coil on the few occasions that it is necessary can be done manually.
2. Attenuation (d): The attenuation factor can also be expressed as the quotient of I.sub.R /I.sub.c, where I.sub.R is the resistive component of the earth fault current and I.sub.c is the capacitive component. I.sub.R is the result of various losses in the system, of which leakage losses across the surface of insulators generally constitute the major part. Leakage losses increase as the amount of dirt on the insulator surfaces increases. In areas with low levels of precipitation the gradual accumulation of dirt on the surface of insulators can give rise to severe operational problems. In such cases, continuous monitoring of the attenuation factor provides a means of improving the efficiency of preventive maintenance. In combination with the degree of mistuning, the attenuation factor can also be used for controlling compensation devices in accordance with Swedish Pat. No. 8401365-5. By means of this, or similar devices, it is possible to compensate both the reactive and the active components in the earth fault current. Here the NX-analyser creates the conditions for automatic control.
3. Degree of dissymmetry (k): Continuous monitoring of the degree of dissymmetry, and in particular of its relative changes, provides a highly sensitive criterion for detection of various fault states in the power system. In particular, the ability to detect high-impedance earth faults can be mentioned. The total degree of dissymmetry, k.sub.total, of the power system consists of the degrees of dissymmetry k.sub.L1, k.sub.L2 etc. of the individual line sections. These, in turn, are very characteristic and arise as a result of the difference between the capacitive couplings of the phase conductors to earth as a result of their geometrical positions. Switching in or out individual lines thus also changes the calculated total degree of dissymmetry as determined by the NX-analyser, which restricts the practical feasibility of making direct use of k.sub.total or of its relative changes .DELTA.k.sub.total as an earth fault criterion.